It is high time we turned our attention to a complex mathematical problem. Namely, how FunCPU computes:
1+1=?
Please recall that the definition of "add" is stored at 00 (referenced by 81) is as follows:
add(x,y):= FD 7F 7E FC 81 7E FE 7F FF
The following is an extract from the FunCPU expression memory, which clearly indicates how a simple expression is stored, evaluated and reduced to a final result.
: 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
00: 81 01 01 FF FD 01 01 FC 81 01 FE 01 FF FC 81 01
10: 00 FF FC FD 01 00 FC 81 00 FE 01 FF FC FC 81 00
20: 00 FF FC FC FD 00 00 FC 81 00 FE 00 FF FC FC 00
30: FF FC 01 FF 02 FF .............................
Where different colors denote different expressions as illustrated below:
- Initial expression 1+1
- the result
- first instance of unfolded, bounded function definition
- third cyle, after "if" condition is evaluated
As the above extract reveals, in the first cycle, the function definition is unfolded with the arguments bounded simultaneously. Then the "if" expression is evaluated. Since second argument is not zero, the else part is copied to the third cycle. Note, that the second argument now is reduced, thus add(1,0) is called. The result of this call is incremented, thus effectively expression became 1+add(1,0).
The following extract depicts the state of the expression memory at the final stage, when fac(5) is calculated. Result is shown in green. One can see, that evaluating fac(5) has required quite a many cycles (this is basically due to the low level of efficiency of the built-in functions inc and dec), and the original expression calling fac(5) is overwritten.
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
00: FF FC FC FC 75 FF FC FC 76 FF FC 77 FF 78 FF FC
10: FC FD 03 71 FC 81 71 FE 03 FF FC FC FC FC FC 81
20: 71 02 FF FC FC FC FC FC FD 71 02 FC 81 02 FE 71
30: FF FC FC FC FC FC FC 81 02 70 FF FC FC FC FC FC
40: FC FD 02 70 FC 81 70 FE 02 FF FC FC FC FC FC FC
50: FC 81 70 01 FF FC FC FC FC FC FC FC FD 70 01 FC
60: 81 01 FE 70 FF FC FC FC FC FC FC FC FC 81 01 6F
70: FF FC FC FC FC FC FC FC FC FD 01 6F FC 81 6F FE
80: 01 FF FC FC FC FC FC FC FC FC FC 81 6F 00 FF FC
90: FC FC FC FC FC FC FC FC FD 6F 00 FC 81 00 FE 6F
A0: FF FC FC FC FC FC FC FC FC FC FC 81 00 6E FF FC
B0: FC FC FC FC FC FC FC FC FC FD 00 6E FC 81 6E FE
C0: 00 FF FC FC FC FC FC FC FC FC FC FC 6E FF FC FC
D0: FC FC FC FC FC FC FC 6F FF FC FC FC FC FC FC FC
E0: FC 70 FF FC FC FC FC FC FC FC 71 FF FC FC FC FC
F0: FC FC 72 FF FC FC FC FC FC 73 FF FC FC FC FC 74
